This invention relates to high-strength solutions of potassium hydrogen peroxymonosulfate that are low in inert salt content, a process for their preparation, and their use in the manufacture of printed wiring boards and semiconductors.
Trademarks are hereinafter shown in upper case.
The term xe2x80x9cpotassium monopersulfatexe2x80x9d, is commonly used in the trade, and is used herein to refer to the mixed triple salt 2KHSO5.KHSO4.K2SO4. Potassium monopersulfate has a theoretical active oxygen content of 5.2%. All references to potassium monopersulfate herein refer to the commercially available mixed triple salt which has a typical active oxygen content of about 4.7%. It is recognized that the purity of commercially available potassium monopersulfate is about 95% to about 98% due to the presence of minor impurities, minor amounts of additives (such as an anticaking agent), and variations in manufacturing processes.
The term xe2x80x9cKHSO5xe2x80x9d, the formula of the active component in the triple salt, is used herein specifically to denote potassium hydrogen peroxymonosulfate itself.
The production of modern printed wiring boards, also known as printed circuit boards, for developing the desired electrically conductive pattern onto plastic or other substrates clad with copper foil is a complex process. In the process, electrical interconnections are made by selectively removing unwanted copper foil or adding additional copper to the substrates. The process requires careful removal of excess copper, reagents remaining from the copper deposition, powder, dust, oxides, and other debris or residuals after each construction step. It is not unusual for the process to require 100 or more individual steps because of the complexity of electrical circuitry configurations.
The removal of residual copper is termed xe2x80x9cmicroetchingxe2x80x9d to emphasize the controlled nature of the action. Careful and gentle removal is required, as contrasted with energetic and bulk removal of copper by the use of highly active chemicals, sometimes under extreme conditions of temperature, pH and/or modes of application. The microetching must be combined with cleaning action for efficient and economical processing. Microetching copper is a critical step in manufacturing printed wiring board""s because it ensures that plating or resist layers will adhere to the substrate each time they are applied. The rate of residual copper removal by application of the microetchant is termed xe2x80x9cetch ratexe2x80x9d.
The microetching process involves contacting the printed wiring boards with the potassium monopersulfate solution (typically as a bath), microetching and cleaning of copper clad printed wiring board substrates, and removing the solution together with undesired residues. In the use of the microetching bath, copper is etched and the KHSO5 active ingredient is consumed. Simultaneously, the KHSO5 forms sulfate salts as the active oxygen is consumed, adding to the sulfate concentration formed from the potassium sulfates present in the potassium monopersulfate. Consequently, copper sulfate concentration in the bath increases, and eventually the bath must be replaced, else staining of the boards occurs and quality declines. While additional potassium monopersulfate may be added to supplement decreasing active oxygen, sulfate salts build up, limiting the useful life of the bath.
It is desirable to improve the microetching process through use of microetching solutions providing lower inert sulfate concentrations and higher weight percent active oxygen. The present invention provides such solutions.
The present invention comprises a composition comprising a solution of potassium hydrogen peroxymonosulfate having a minimum weight ratio of SO5 to SO4 of greater than 1.0:1.
The present invention further comprises a composition comprising a solution of potassium hydrogen peroxymonosulfate having a minimum active oxygen content, at an equivalent temperature, equal to from about 1.5 to about 5.5 times the active oxygen content of 2KHSO5.KHSO4.K2SO4 when said 2KHSO5.KHSO4.K2SO4 is completely dissolved.
The present invention further comprises a process for the preparation of a solution of the above potassium hydrogen peroxymonosulfate comprising a) mixing solid 2KHSO5.KHSO4.K2SO4 with a quantity of water insufficient to completely dissolve the solid, b) mixing to form a slurry, and c) separating undissolved solids to obtain the solution.
The present invention further comprises an improved method of microetching a surface wherein the improvement comprises contacting the surface to be etched with a solution of potassium hydrogen peroxymonosulfate having a minimum ratio of SO5 to SO4 of greater than 1.0:1.